1. Field of the Invention
The present invention relates to a laser cutting method that is used preferably for performing cutting processing on laminated substrates and the like, a method for manufacturing a display apparatus using the laser cutting method, and a display apparatus manufactured by employing the laser cutting method.
2. Description of the Related Art
Recently, a liquid crystal display apparatus has rapidly become widespread as a small-sized display apparatus for a projector apparatus, portable telephone, and the like by making the best use of its characteristics, i.e. it is capable of saving the power consumption, reducing the weight, or sliming the shape.
The liquid crystal display apparatus uses a liquid crystal as a light control element by controlling the transmittance of light through applying an electric field to the liquid crystal, thereby controlling the contrast to display an image. This liquid crystal is sandwiched between a stripe-type scanning electrode group and a stripe-type signal electrode group that is orthogonal to the scanning electrode group, and a voltage is applied to the intersections of those electrodes to drive the liquid crystal. This method is called a simple matrix drive. With this simple matrix drive, it is not possible to obtain a high display quality, and there is a limit set in the number of scanning lines.
In order to improve the aforementioned performance of the simple matrix drive, an active matrix system that has a switching element provided at each pixel is used widely. In particular, recently, a liquid crystal display apparatus that uses a thin film transistor for the switching element has become popular in general. As a material for forming the thin-film transistor, there is amorphous silicon. The thin-film transistor using amorphous silicon has been widely used conventionally, since it can be formed at low cost and can be easily formed in a large scale.
However, the amorphous silicon thin-film transistor has such a defect that the mobility of the electric charges is low. Thus, it is necessary to mount a driving circuit for driving the thin-film transistors in the vicinity of a display panel. Therefore, there is a limit in reducing the size of the liquid crystal display apparatus, and it has become unsuitable for the display apparatus of a portable telephone and the like, which requires reduction in the size, weight, and sliming the shape.
Upon this, a liquid crystal apparatus that uses a polycrystalline silicon thin-film transistor with high carrier mobility has become popular as a small-sized liquid crystal display apparatus. In the liquid crystal display apparatus using the polycrystalline silicon thin-film transistors, peripheral driving circuits can be formed on a same substrate. Thus, it is effective for narrowing the frame when a display area is narrow.
For improving the productivity of the liquid crystal display apparatuses, it is common to cut and separate the laminated substrates into individual display apparatuses, after laminating a counter substrate on which color layers are formed and a driving substrate that is a large-scaled glass substrate on which patterns of display apparatus constituted with a plurality of pixel driving thin-film transistors and driving circuits are formed. As a cutting method thereof, there is a way in which scribe lines are formed on the substrate by a diamond cutter, and then it is broken from the rear face.
However, with this cutting method using a cutter, chippings are generated on the cut face so that it becomes fragile. Therefore, the panel strength is deteriorated, thereby causing such issues that it is necessary to be washed and polished during the manufacturing step, etc.
As a method for improving such issue, Japanese Unexamined Patent Publication 05-305467 discloses a cutting method, in which: paint that absorbs laser beams is applied to the areas that correspond to the cutting position of a substrate; and a laser with a wavelength that transmits the substrate is irradiated from the opposite side of the surface where the paint is applied so as to generate a thermal stress in the irradiated areas for cutting the substrate. This laser cutting method is a noncontact cutting method, so that chippings are not generated in the cut face. Thus, a panel with a smooth cut face can be obtained.
This laser cutting method disclosed in Japanese Unexamined Patent Publication 05-305467 is designed for cutting a single glass substrate. Thus, there are following issues generated when cutting laminated substrates.
When laser beams are irradiated from the opposite side of the paint-applied face of one of the substrates, the laser beams are absorbed by the paint on one of the substrates. Thus, the laser beams are not irradiated to the paint of the other substrate that opposes to this substrate. Therefore, laser beams need to be irradiated separately to each of the substrates, thereby decreasing the throughput. Further, it requires a step for applying the paint, so that the productivity is decreased.
Further, with the conventional method using laser beams, as shown in FIG. 24A, it is necessary to irradiate laser beams separately from both sides, i.e. from the side of one glass substrate 101 and the side of other glass substrate 102. In this case, as shown in FIG. 24B, the laser-beam irradiated areas of the substrate glasses 101 and 102 are heated first. Then, as shown in FIG. 24C, cracking is generated in the heated areas. At last, there may cause such an inconvenience that the directions of each cracking are shifted as shown in FIG. 24D (it is cooled in a gap retaining member 103 in the center). Thus, the cutting accuracy becomes bad.